Direct-fired kiln furnace control system

ABSTRACT

A direct-fired kiln furnace control system for lumber and like material utilizing a natural gas or oil burner with provision for spraying water into the furnace gases for humidification purposes wherein the furnace operation is controlled in response to not only a pair of sensor signals reflecting the dry bulb and wet bulb conditions in the kiln proper, but also in response to a sensor signal reflecting the temperature of the gases at the outlet of the furnace, so as to thereby insure the rapid, safe and effective vaporization of water introduced into the furnace gases. The three sensor signals are utilized to selectively control the combustion rate of the furnace burner, as well as a combination of vent, damper and air fan means regulating the flow rate and recirculation of the heated gases through the kiln furnace, to thereby achieve and maintain required temperature and humidity conditions in the kiln.

United States Patent Inventor Walker L. Welliord, Jr. Memphis, Tenn.

Appl. No. 876,733 Filed Nov. 14, 1969 Patented Oct. 19, 1971 AssigneeMoore Dry Kiln Company of Oregon North Portland, Oreg.

DIRECT-FIRED KILN FURNACE CONTROL SYSTEM 13 Claims, 3 Drawing Figs.

US. Cl 263/40 R, 34/46, 263/19 A Int. Cl F27b 3/02 Field of Search263/19 A, 40 R; 34/46 References Cited UNITED STATES PATENTS 8/! 952Foulder et al 34/46 3,269,715 8/1966 Wellford,Jr

Primary Examiner-John .l. Camby Attorney-Daniel P. Chernofi ABSTRACT: Adirect-fired kiln furnace control system for lumber and like materialutilizing a natural gas or oil burner with provision for spraying waterinto the furnace gases for humidification purposes wherein the furnaceoperation is controlled in response to not only a pair of sensor signalsreflecting the dry bulb and wet bulb conditions in the kiln proper, butalso in response to a sensor signal reflecting the temperature of thegases at the outlet of the furnace, so as to-thereby insure the rapid,safe and effective vaporization of water introduced into the furnacegases. The three sensor signals are utilized to selectively control thecombustion rate of the furnace burner, as well as a combination of vent,damper and air fan means regulating the flow rate and recirculation ofthe heated gases through the kiln furnace, to thereby achieve andmaintain required temperature and humidity conditions in the kiln.

PAIENTEnum 19 mm SHEET 1 [IF 3 DIRECT-FIRED KILN FURNACE CONTROL SYSTEMBACKGROUND OF THE INVENTION This invention relates to improved controlapparatus for a direct-fired gas or oil-fueled kiln furnace of the typewherein water is added to the hot mixture of combusted furnace gases andrecirculated air from the kiln during certain intervals of the dryingand conditioning cycle to provide a circulating gaseous medium ofdesired temperature and moisture content for treating material containedin the kiln. More particularly, the invention relates to improvedcontroller means for automatically regulating the operation of adirect-fired kiln furnace of the type described in a more advantageousand efficient manner, especially during those periods in the dryingcycle when increased moisture content is required in the circulatinggases and, in response thereto, a large quantity of water in liquid formis introduced for vaporization in the combusted furnace gases. While thecontrol apparatus of the present invention may be advantageouslyemployed in furnace systems supplying a heated and moisturized gaseousmedium to kilns for treating various kinds of material, it isparticularly suitable for application in dry kilns used for the dryingand conditioning of hardwood lumber. Accordingly, the invention will bedescribed and illustrated herein as applied to dry kilns of this type.

It is common practice in the kiln drying of materials such as lumber tosupply heated air which has been heavily laden with moisture forconditioning the lumber properly during the drying process in orderthereby to prevent over-rapid drying of the surface and exteriorportions of the lumber which otherwise might cause warping, checking orother forms of deleterious damage or degradation of the product. In akiln furnace of the direct-fired type the gaseous drying medium isobtained by vaporizing water, as needed, into combusted furnace gasesproduced by an oil or gas-fueled burner and thereafter circulating theresulting mixture through the kiln system. In two of my prior U.S. Pat.Nos. 3,269,715 and 3,151,850, I disclosed direct-fired kiln furnacesystems of this general type containing associated controller means forregulating the operation of the furnace and the introduction of waterspray so as to maintain predetermined temperature and humidityconditions in the kiln. As pointed out in the aforesaid prior patents,one of the major concerns in operating a kiln system of the directfiredtype, in contrast to the more conventional kiln designs in which steamobtained from a separate boiler is circulated through the kiln as amoisturizing medium to dry and condition the product, is the requirementthat substantially all of the water injected into the combusted furnacegases be quickly and completely vaporized into the circulating airstream; otherwise, the unvaporized water tends to deposit on the wallsand floor of the kiln, producing scaling and other corrosive effects, oron the lumber itself, leaving undesirable lime deposits and stain.

The aforesaid U.S. Pat. Nos. 3,269,715 and 3,151,850 were directed toforms of dual-mode controllers for such directfired kiln furnace systemsin which rapid and complete evaporation of the water introduced into thecirculating gaseous medium during certain periods in the drying cyclewas achieved by operating the furnace burner at a higher combustion rateduring such periods, regardless of whether or not additional heat demandwas called for by the dry bulb conditions then present in the kilnchamber proper. Thus, even during periods when the kiln environmentmight be at the desired dry bulb temperature level so that theinstantaneous heat demand from the kiln furnace would be relatively low,if additional moisture content were needed in the circulating gases thefurnace controller would operate to increase the burners combustion rateduring such periods so as to insure that sufficient caloric energy wasavailable in the furnace gases to rapidly and effectively vaporize thewater as it was introduced into the gas stream.

SUMMARY OF THE INVENTION In practice, in the operation of direct-firedkiln furnace systems of the type disclosed in the aforementionedpatents, it has been found that more satisfactory and completeevaporation of the introduced water, together with improved control overthe temperature and humidity conditions pertaining in the kiln, could beeffected if additional provisions were made for regulating thetemperature and flow rate of the circulating kiln gases. Accordingly,innovations have been made to the basic direct-fired kiln furnacecontrol systems of the aforesaid prior patents so as to achieve the dualobjectives of 1) better evaporation of the water spray introduced intothe furnace gases during periods when added moisture content is neededin the kiln conditioning medium, and (2) improved efficiency andresponse time of the system in reaching and maintaining prescribedtemperature and humidity conditions in the kiln environment duringvarious stages of the drying cycle.

In the direct-fired kiln furnace system of the present invention, acombination of vents, dampers and, in some cases, variable speed air fanelements are provided which, in response to actuating signals receivedfrom a controller unit, regulate the circulation and recirculation ofcombusted furnace gases through both the kiln furnace and the remainderof the kiln system in a manner so as to obtain rapid and effectiveadjustment to changes in the dry bulb or wet bulb temperature needs inthe kiln chamber. Through selective actuation of the vents, dampers andair fan elements, the flow rate of the air mass circulating through thekiln system, and the portion of that air flow which is recirculatedthrough the system as tempering air, are varied in response to theinstantaneous temperature and humidity demands of the kiln as determinedfrom comparison with prearranged settings.

Control of the flow rate of the gaseous air medium circulated throughthe kiln, together with the control over the percentage of heatedtempering air recirculated through the kiln furnace, provide effectivemeans for controlling the kiln environmental conditions. In particular,during periods when water is being added to increase the moisturecontent of the conditioning gases circulating through the kiln system,the flow rate of the furnace gases is decreased in conjunction with anincrease in the relative percentage of tempering air recirculatingthrough the furnace. In this manner the temperature of the furnace gasesmay be substantially increased to a level sufficiently high toeffectively vaporize substantially all of the injected water spraywithout altering the combustion rate of the burner, the latter beingcontrolled solely in response to the dry bulb temperature demandrequirements present in the kiln chamber. Even though the temperature inthe kiln furnace rises sharply during such periods, the reduced flow ofair into the kiln chamber proper ensures that the dry bulb temperaturewill not overshoot but instead will be maintained at its prescribedlevel. The foregoing arrangement is highly efficient as it minimizes thecaloric heat demand required from the furnace burner to both maintaindesired dry bulb temperature conditions in the kiln chamber and to alsosatisfactorily vaporize water introduced into the furnace combustionchamber during periods when the circulating kiln gases require increasedmoisture content.

In order to obtain safe yet effective regulation of the temperature andhumidity conditions in the gaseous medium circulating through the kilnfurnace and kiln chamber, the controller unit, which controls thefurnace burners combustion rate and actuates the vents, dampers and airfan components controlling the circulation of the drying andconditioning gases through the kiln system, is responsive not only tothe dry bulb and wet bulb temperatures present in the kiln chamberproper, but the unit also receives a third sensory input responsive tothe temperature conditions pertaining at the outlet duct or exhaust ofthe furnace which couples to the kiln chamber. This last-mentionedsensor, which typically registers a reading several hundred degrees(Fahrenheit) hotter than the dry bulb temperature of the gasescirculating in the kiln area itself, is utilized to prevent operation ofthe kiln furnace in an overheated condition above a safe temperaturelevel. The signal received from the outlet duct temperature sensorserves as a limit signal, to override the other control elements in thesystem which regulate the burner combustion rate and the flow of thecirculating gases through the kiln, so as to maintain the furnacetemperature within a safe operating range below a predetermined dangerthreshold. By reason of the comprehensive and cooperative regulationprovided over the burner combustion rate and the circulating gaseousflow through the kiln, together with the override safety featureprovided by the sensor monitoring the temperature of the furnace gases,the kiln furnace control system of the present design is capable ofrapidly yet safely adjusting to meet changed temperature and/or humiditydemand conditions in the kiln.

It is therefore a principal objective of the present invention toprovide an improved direct-fired kiln furnace system of the type whereinwater in liquid form is vaporized into the furnace gases to provide aheated and moisturized gaseous medium for treating kiln material.

It is another objective of the present invention to provide an improvedcontroller circuit for a kiln furnace system of the type describedwherein the system is operated in a more economical, efficient and safemanner by reducing the amount of caloric heat energy required to rapidlyand effectively vaporize water introduced into the furnace gases and bytempering and regulating the flow rate of the furnace gases circulatingthrough the kiln system.

The foregoing and other objectives, features and advantages of thepresent invention will be more readily understood upon consideration ofthe following detailed description of an illustrative embodiment of theinvention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a front elevational view,partly sectional and partly schematic, of an illustrative type ofdirect-fired kiln furnace which may be advantageously used with theimproved controller apparatus of the present invention.

FIG. 2 is a block diagram of a kiln furnace system employing theimproved controller of the present invention for providing automaticoperation and regulation of a direct-fired kiln furnace.

FIG. 3 is a schematic circuit diagram of an illustrative embodiment ofan electropneumatic controller unit for use with the kiln furnace systemof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a direct-fired kiln furnace, designated generally as 20,connected to a lumber kiln of conventional design. The furnace walls maybe lined with refractory material or any suitable high-temperaturescale-resisting alloy such as nickel, chromium and iron. In thecombustion region A of the furnace gas or oil fuel supplied from asource (not shown) via a pipe line 32 to burner 30 is intermixed andcombusted with a quantity of air forced at high velocity over air line34 by blower fan 36. The combustion rate of the fired gases is varied inconventional manner by means of a pneumatically actuated valve 38controlling the air-fuel mixture by regulation of the rate of air flowin the air line 34.

The combusted hot gases, fired by the burner 30 in combustion portion Aof the furnace 20, then flow out through passageway 22 into region B ofthe furnace where water is evaporated therein, if needed, to increasethe moisture content of the drying medium prior to its circulationthrough the kiln chamber. In the vaporization region B water from apressurized source (not shown) is supplied over water line 92 and undercontrol of valve 94 to nozzle 91 which disperses the water into a spray90 of fine droplets. The water spray is directed within the region B soas to thoroughly intermix with the combusted hot gases emergent from thecombustion section A of the furnace. The small amount of water spraywhich is not immediately vaporized by the heat of the gases is carriedaway by a drain 98 and, if desired, it may be returned to a sump pumpfor recirculation. An eliminator 101 is preferably provided in thepassageway 22, as far downstream from the spray nozzle 91 as ispractical, for precipitating out the unvaporized particles of waterentrained in the air stream while permitting the moisturized heated airto pass freely therethrough. The moisturized gases then exit from thefurnace, through an outlet duct 24 connecting to the kiln chamberproper, where they serve as the drying and conditioning medium for thematerial under treatment in the kiln 10.

After circulation within the lumber kiln the gas is drawn therefrom bysuitably located ducts (not shown) and directed into a passageway 40which couples to the gas-fired furnace 20. Control of the inlet air tothe furnace is provided by a pair of vents 42 and 44, which may belocated along the passageway 40 as indicated in FIG. 1 or alternativelyon the roof or sidewalls of the kiln chamber 10, and a damper element 46positioned in the passageway 40 across thepath of the air streamexhausting from the kiln. The vent and damper elements are ofconventional design with vanes adjustable from a fully opened to a fullyclosed position. The combination of the vents 42, 44 and the damper 46permits complete regulation of the proportion of tempering return airfrom the kiln chamber 10 in relation to the proportion of fresh ambientair introduced for circulation in the kiln furnace system. For example,it will be seen that with damper 46 fully closed and vents 42 and 44fully open, the exhausted kiln gases are expelled to the atmosphere, andall of the air supplied to the inlet of the kiln furnace is in the formof fresh cold air drawn from the outside. On the other hand, with damper46 fully opened and vents 42, 44 fully closed, all of the air suppliedto the kiln furnace is in the form of recirculating gases from the kilnchamber 10. In any intermediate position of the ventdamper combinationthe air supplied to the inlet of the kiln furnace 20 comprises a mixtureof tempering air with cold air drawn from the outside, with the relativepercentages of each component determined by the respective settings ofthe vanes in the circulation control elements.

Proceeding downstream in the passageway 40 from the vent dampercombination, an air fan is provided at the inlet to the furnace chamber20 which is driven by a variable speed motor 82. (In lieu of utilizing avariable speed motor, the motor drive 82 could remain constant and thefan 80 instead provided with propeller blades of adjustable pitch). Thespeed (or pitch) of the air fan 80 is regulated so as to permit the flowrate of the gaseous medium circulated through the kiln furnace and intothe kiln chamber to be varied in accordance with control signalssupplied thereto, as will be hereinafter described.

As an alternate to, or in conjunction with, the control of thetemperature of the circulating kiln gases effected by regulation of thegas flow rate with the air fan 80, a bypass passageway 41 may beprovided, as shown, connecting the post-combustion region B of thefurnace to its air inlet downstream from the vent-damper combination.This bypass passageway 41 is provided with a damper 48 which, whenopened, permits the shunting of a portion of the combusted hot gasesemergent from combustion region A into the inlet passageway 40 forrecirculation through the furnace. Through internal recirculation inthis manner, the temperature of the gas mixture formed in the furnacecan be quickly raised without need for altering the combustion rate ofthe furnace burner 30.

With the capability provided by the aforementioned ventdamper, fan andbypass means for increasing the temperature of the furnace gases throughregulation of their flow rate and recirculatory flow paths, a rapid andsubstantially complete vaporization of the water spray introduced by thenozzle 91 at region B can be achieved, even though there be at that timea relatively low calorie heat demand required from the furnace tomaintain prescribed dry bulb temperature conditions in the kiln.

FIG. 2 is a block diagram of the aforedescribed kiln furnace systemincorporating the improved controller apparatus of the presentinvention. This kiln control system provides automatic operation andregulation of (l) the kiln furnace combustion rate, (2) the water spraymeans for evaporating moisture into the kiln gases as needed, and (3)the circulatory flow and composition of the furnace air. Thedirect-fired furnace 20, whose construction is shown in detail in theprevious figure, is illustrated in a schematic representation asjuxtaposed next to a kiln chamber of conventional design. A dry bulbtemperature sensor 65 and a wet bulb temperature sensor 60 are suitablylocated within the kiln 10 for monitoring, respectively, the temperatureand humidity of the drying medium circulating within the kiln. Thesignal indications from the dry and wet bulb thermometers are suppliedover respective lead wires 67, 62 to a heat and humidity regulator 100.100. As more particularly shown in FIG. 3, the heat and humidityregulator 100 may be of conventional design, such as for example aFoxboro vacuum pneumatic temperature-humidity recording controller orother suitable type wherein an pneumatic output signal is derivedresponsive to the difference between the input signal, corresponding tothe actual measured dry bulb or wet bulb temperature parameter, and apredetermined value of temperature or humidity preset in the regulator.Thus, referring again to FIG. 2, two pneumatic output signals aresupplied from the regulator 100: the first is a Heat Signal, appearingon lead line 102, which represents the difference between the dry bulbtemperature present within the kiln 10 and the desired temperature asset on the regulator; and the second output is a Humidity Signal,appearing over lead line 104, which similarly represents the differencebetween the actual and desired wet bulb temperature or moisture contentof the drying medium in the kiln. These two signals, together with athird temperature signal which is provided over lead line 72 by athermocouple 70 located in the outlet duct 24 of the furnace formonitoring the temperature of the furnace exhaust gases, are supplied asinputs to a controller unit 110 which controls and regulates theoperation of the kiln furnace system.

The controller unit 110 has outputs 120, 130, 140 and 150 forcontrolling the operation respectively of the vent positioners 125a,1256, the servomotor 135 positioning dampers 46 and 48 and the variablespeed motor 82 driving fan 80; the burner air control 38; and the sprayvalve 94. Through regulating of the aforementioned elements, in responseto heat and humidity signals supplied to the controller, the temperatureand air circulation through the kiln furnace are controlled so as toreach and maintain desired dry bulb and wet bulb temperature conditionsin the kiln chamber 10 in a quick and highly efficient manner.

Referring now to FIG. 3, there is illustrated an electropneumaticschematic diagram of the construction and circuit configuration of thecontroller unit 110. The pneumatic Heat Signal 102 and Humidity Signal104, together with the signal provided over lead line 72 from the sensor70 representing the temperature at the outlet duct of the furnace, aresupplied as inputs to the controller 110. The Humidity Signal 104,responsive to any instantaneous deviation existing between the presetand the actual wet bulb temperature in the kiln, is utilized to actuatevia line 150 the spray control valve 94 thereby introducing water sprayinto the combusted furnace gases for increasing the moisture content ofthe drying medium. During such periods, when water is being added to thefurnace gases, the vent elements 42, 44 are also being positioned towarda closed position by respective pneumatically actuated motors 1250, 125kacting in response to the Humidity Signal 104 appearing over line 120.Closing of the vanes of these vents decreases the proportion of freshair introduced into the kiln, while at the same time increasing theproportion of heated return air from the kiln chamber which is availablefor recirculation within the kiln system. Also during such periods whenwater spray is being introduced into the kiln furnace gases, thetemperature of the furnace gases may be further raised by having thedamper element 46 biased towards the closed position, the bypass damper48 biased toward the open position, and the speed of the air fan in thekiln furnace decreased, all in response to actuating signals receivedover line in a manner hereinafter to be explained.

The Heat Signal 102, the magnitude of which represents the differenceexisting between the desired and actual dry bulb conditions in the kilnchamber, is utilized to regulate the operation of the furnace burner andalso, together with the Humidity Signal, to regulate the air circulationthrough the kiln furnace system. The Heat Signal is supplied as an inputboth to a limit controller element 106 and also to differential pressurecontrol element 108. The limit controller 106, which receives a controlsignal on line 72 representing the temperature of the gases detected bysensor 70 in the outlet duct of the furnace, acts as a gating unit toblock passage of the Heat Signal 102 therethrough when the furnace gastemperature exceeds a predetermined safe limit, for example 450 F. Thus,when the temperature of the furnace gases are within a safe operatingrange beneath the 450 F. level, the pneumatic Heat Signal 102,proportional to the added caloric heat demand required from the furnace,passes through the limit controller element 106 without diminishment inamplitude. However, as the temperature of the furnace gases approach the450 F. threshold level, the element l06 commences to act as a shutoffvalve, decreasing the amplitude of the pneumatic signal appearing on theoutput line 140.

The signal derived from the limit controller 106 and appearing on lineis used for two purposes. Firstly, the signal 140 is employed toregulate the operation of the air supply to the burner 30 by actuationof the pressure-responsive air valve 38, thus controlling the combustionrate of the furnace. In this way the furnace burner is regulated inresponse to dry bulb temperature conditions in the kiln chamber, thecontroller acting to turn up the burner during periods when added heatis required and to decrease it when the actual dry bulb temperatureconditions in the kiln chamber are at or near the desired level set onthe heat-humidity regulator 100. The output 140 of the limit controlleris also supplied as a second input to the differential pressure controlelement 108 which, as previously mentioned, receives as its other inputthe Heat Signal 102 derived from the regulator 100. So long as thetemperature of the furnace gases remain within the predetermined safeoperating range, as monitored by the outlet duct temperature sensor 70,the limit controller element 106 will pass through the Heat Signal 102unimpeded, and thus the respective inputs to the differential pressurecontrol element 108 will be equal, producing a zero or minimum signal onthe output line 112. However, as the temperature of the furnace gasesapproach the predetermined limit (450 F.), the action of the limitcontroller element 106 causes a differential pressure to exist betweenthe respective inputs 102 and 140 to the control element 108, therebyproducing a pneumatic pressure signal on the output line 112. Thepresence of a signal on line 112, representing a condition in which thetemperature of the furnace gases has risen to the limit of the safeoperating range concurrently with a demand that the kiln furnace providegreater heat content in the circulating kiln gases in order to achievedesired dry bulb temperature conditions in the kiln chamber, is suppliedto' a restrictor tee 114 controlling a supply line 116 of pressurizedair.

The controlled air supply line 116 as well as the Humidity Signal 104are both supplied as inputs to a blocking relay element 118 whichperforms the function of passing through to its output line 130 thegreater of the two input signals. Thus, in this respect, relay element118 functions similarly to a logical OR device to pass through thesignal of greater amplitude as between the respective inputs 116 and104. The amplitude of the control signal appearing on line 130 is usedto regulate the positioning of the respective dampers 46 and 48 in theinlet and bypass passageways of the furnace as well as the motor speedof the air fan 80. In this fashion, the output signal 130 from thecontroller unit 110 regulates both the flow rate and the circulatorypath of the gases traveling through the kiln furnace and the kilnchamber. Thus, with a minimum or zero level output on line 130, thedampers 46 and 48 are both biased towards the closed position and thisarrangement, in conjunction with the concurrent positioning of vents 42and 44 towards the open position, produces a circulatory path for thekiln furnace system in which very little if any of the furnace gases areeither bypassed internally or recirculated as tempering air, so thatvirtually all of the air is drawn fresh from the outside and thenexhausted upon a single pass through the kiln chamber. Under theforegoing condition of operation, with the dampers 46 and 48 biased totheir closed positions, the presence of the minimum amplitude signal onoutput line 130 controls the operation of motor 82 so as to produce amaximum speed of revolution for the circulatory air fan 80, thusproviding a high rate of flow for the movement of the heated furnacegases through the kiln system Upon the occurrence of a Humidity Signal104 representing the need for water to be added to the furnace gases toincrease their moisture content, or upon the occurrence of a Heat Signal102 representing the need for added caloric content in the furnace gasesto increase the dry bulb temperature conditions in the kiln, or upon theoccurrence of both conditions, the controller output signal appearing online 130 regulating the positioning of the dampers and the speed of theair fan will be the greater of the respective amplitudes of the inputsignals to the relay 118. Thus, the signal of greater amplitude willoperate to bias the respective dampers 46 and 48 towards their openpositions and to decrease the speed of, or even shut off, the fan 80,these actions having the joint effect of increasing the relativepercentages of tempering and fumace-bypass air recirculated through thefurnace and at the same time decreasing the flow rate of the furnacegases.

As previously mentioned, the foregoing operations will serve tosubstantially increase the temperature of the furnace gasesindependently of any temperature increase produced through regulation ofthe combustion rate of the furnace burner. Consequently, any increase inthe amplitude of the output signal appearing on line 130, responsive tothe presence of extra heat or humidity demand for the furnace gases,will produce corresponding changes in the settings of the respectivedampers and fan speed so as to increase the temperature of the furnacegases and thereby increase the temperature of the circulating kiln gasesor insure effective vaporization of the injected water spray, as thecase may be. However, in the event that the temperature of the furnacegases should approach the safe operating limit (e.g., 450 F.) preset inthe limit controller 106, this element, acting through the medium of thedifferential pressure control element 108 and the restrictor tee 114,reduces the amplitude of the signal 116 supplied as an input to therelay 118. This decrease in input amplitude in turn is reflected as asignal of decreased amplitude on controller output line 130, thus movingthe dampers 46 and 48 towards the closed position and increasing thespeed of the fan motor 82 so as to thereby produce both a decrease inthe percentage of tempering air as well as an increase in thecirculatory flow rate. These adjustments result in a consequentreduction in the temperature of the furnace gases so as to restoreoperation within the safe temperature range.

Control valve 115 is preferably provided in the line 116 supplying thepressurized air signal to the relay 118 so that, during startup of thekiln furnace system, this line could be blocked off thus insuring thatthe dampers and fan control will be maintained during this initialperiod at settings providing maximum circulation flow of the furnacegases through the kiln system.

In some embodiments of kiln furnace systems constructed in accordancewith the teachings of the present invention provision for regulating thespeed of the circulatory air fan 80 can be dispensed with, as adequatecontrol over the kiln gas temperature and flow conditions can beachieved through the regulation provided by the combination of vents anddampers determining the percentage of tempering and internallyrecirculated air introduced into the furnace. In a further variation ofthe exemplary kiln furnace controller disclosed herein, the positioningof the respective vents 42 and 44 could also be regulated by the signalproduced on the output line from the blocking relay 118, rather thandirectly from the Humidity Signal 104 as shown in FIG. 3, so thatcontrol of the vents, as well as the damper and fan elements, would bein response to the greater of the respective signals reflecting heat andhumidity demands in the kiln chamber, and the operation of these ventswould then also be subject to the override safety control provided bythe outlet duct temperature sensor 70 and its associated controlelements.

The terms and expressions which have been employed in the foregoingabstract and specification are used therein as terms of description andnot of limitation, and there is no intention, in the use of such termsand expressions, of excluding equivalents of the features shown anddescribed, or portions thereof, it being recognized that the scope ofthe invention is defined and limited only by the claims which follow. 4

What is claimed is:

1. In a direct-fired kiln system, the combination comprising a. a kilnchamber for drying and conditioning material,

b. a kiln furnace having an air intake, a burner, and a duct couplingheated furnace gases to the inlet of said kiln chamber,

c. spray means in said kiln furnace for injecting water for vaporizationinto said heated furnace gases,

d. intake air control means for adjusting the relative constituents offresh ambient air and tempering heated air returning from said kilnchamber, the mixture of which constituents is supplied as intake air tosaid furnace, and

e. controller means for regulating the operation of said intake aircontrol means during periods when water is interjected into said heatedfurnace gases by said spray means, whereby the caloric heat content ofsaid furnace gases is maintained at a level sufficient to effect a rapidand substantially complete vaporization of said water spray therein.

2. The kiln system of claim 1 further including a passageway providedwith an adjustable damper coupling said furnace duct directly to saidfurnace air intake for internally recirculating within said furnace aselectable portion of said heated furnace gases, the operation of saidadjustable damper also being regulated by said controller means.

3. In a direct-fired kiln system, the combination comprising a. a kilnchamber for drying and conditioning material,

b. a kiln furnace having a burner for heating gases and a spray meansfor injecting water for vaporization into said heated furnace gases forincreasing the moisture content thereof, said heated and moisturizedfurnace gases being thereafter supplied to said kiln chamber forcirculation therethrough,

c. fan means controlling the rate of flow of said gases through saidfurnace, and

d. controller means for regulating the operation of said fan meansduring periods when water spray is added to said furnace gases, wherebythe caloric heat content of said furnace gases is maintained at a levelsufficient to effect a rapid and substantially complete vaporization ofsaid water spray therein.

4. In a direct-fired kiln system, the combination comprising a. a kilnchamber for drying and conditioning material.

b. a kiln furnace having an air intake, a burner, and a duct couplingheated furnace gases to the inlet of said kiln chamber,

c. spray means in said kiln furnace for injecting water for vaporizationinto said heated furnace gases,

d. an adjustable damper coupling said furnace duct directly to saidfurnace air intake for internally recirculating within said furnace aselectable portion of said heated furnace gases, and

e. controller means for regulating the operation of said adjustabledamper during periods when water is interjected into said heated furnacegases by said spray means, whereby the caloric heat content of saidfurnace gases is maintained at a level sufficient to effect a rapid andsubstantially complete vaporization of said water spray therein.

5. In a direct-fired kiln system, the combination comprising a. a kilnchamber for drying and conditioning material,

b. a kiln furnace having an air intake, a burner, a spray means forinjecting water for vaporization into heated furnace gases forincreasing the moisture content thereof, and a duct for supplying saidheated and moisturized furnace gases to said kiln chamber forcirculation therethrough,

c. fan means controlling the rate of flow of said furnace gases throughsaid furnace,

d. intake air control means for adjusting the relative constituents offresh ambient air and tempering heated air returning from said kilnchamber, the mixture of which constituents is supplied as intake air tosaid furnace,

e. a passageway provided with an adjustable damper coupling said ductdirectly to said air intake for bypassing said furnace chamber andinternally recirculating within said furnace a selectable portion ofsaid heated furnace gases,

f. a pair of sensors positioned in said kiln chamber for monitoringrespectively the instantaneous temperature and humidity conditionsprevailing therein, and

g. controller means connected to receive signals from said sensor pairand acting on said water spray means, said fan means, said intake aircontrol means, and said adjustable damper, whereby the operation of saidkiln furnace is regulated so as to restore and maintain desiredpredetermined temperature and humidity conditions in said kiln chamber.

6. A kiln system according to claim further including a third sensorpositioned in said furnace so as to measure the temperature of saidheated furnace gases and connected as a signal input to said controllermeans, whereby the operation of said fan means, said intake air controlmeans and said damper are regulated so as to reduce the temperature ofsaid heated furnace gases when the signals received from said thirdsensor indicate that the temperature of said gases is closelyapproaching a predetermined maximum level.

7. in a direct-fired kiln system, the combination comprising a. a kilnchamber for drying and conditioning material,

b. a kiln furnace having an air intake, a burner, a spray means forinjecting water for vaporization into heated furnace gases forincreasing the moisture content thereof, and a duct for supplying saidheated and moisturized furnace gases to said kiln chamber forcirculation therethrough,

c. intake air control means for adjusting the relative constituents offresh ambient air and tempering heated air returning from said kilnchamber, the mixture of which constituents is supplied as intake air tosaid furnace,

d. a first sensor located in said kiln chamber for measuring the drybulb temperature of the gases circulating therein,

e. a second sensor located in said kiln chamber for measuring the wetbulb temperature of said circulating kiln gases,

f. a regulator connected to receive signals from said first and secondsensors and generating a pair of heat and humidity" output signalsindicating, respectively, the instantaneous heat and humidityrequirements of said circulating kiln gases as determined by comparisonof said sensor signals with predetermined settings of said regulator,and

g. a controller connected to said regulator containing circuit meanswhich actuate said spray means when increased humidity is needed andwhich regulate the operation of said intake air control means inresponse to the greater of the respective magnitudes of said heat" andhumidity" signals received from said regulator.

8. A kiln system according to claim 7 further including a passagewayprovided with an adjustable damper coupling said furnace duct to saidfurnace air intake for internally recirculating through said furnace aselectable portion of said heated furnace gases, the operation of saidadjustable damper also being regulated by said controller means inresponse to the greater of the respective heat" and humidity" signalsreceived from said regulator.

9. A kiln system according to claim 7 further including a fan meanscontrolling the rate of flow of said furnace gases through said furnace,the operation of said fan means also being regulated by said controllermeans in response to the greater of the respective heat" and humidity"signals received from said regulator.

10. A kiln system according to claim 7 including a third sensorpositioned so as to measure the temperature of said heated furnace gasesand connected as a signal input to said circuit means in said controllerwhereby, in the event the temperature of said heated furnace gasesclosely approaches a predetermined safety threshold, said third sensorsignal acts an an override on the operation of said controller so as toregulate said furnace air intake means in a manner so as to reduce thetemperature of said heated furnace gases.

1!. In a direct-fired kiln system, the combination comprising a kilnchamber for drying and conditioning material,

b. a kiln furnace having an air intake, a burner, a spray means forinjecting water for vaporization into heated furnace gases forincreasing the moisture content thereof, and a duct for supplying saidheated and moisturized furnace gases to said kiln chamber forcirculation therethrough,

means controlling the rate of flow of said furnace gases through saidfurnace,

d. means for controlling the temperature of the intake air supplied tosaid furnace,

a pair of sensors positioned in said kiln chamber for monitoringrespectively the instantaneous temperature and humidity conditionsprevailing therein, and

. controller means connected to receive signals from said sensor pairand acting on said water spray means, said fan means and said intake aircontrol means, whereby the operation of said kiln furnace is regulatedso as to restore and maintain desired predetermined temperature andhumidity conditions in said kiln chamber.

12. In a direct-fired kiln system, the combination comprising a kilnchamber for drying and conditioning material,

b. a kiln furnace having an air intake, a burner, a spray means forinjecting water for vaporization into heated furnace gases forincreasing the moisture content thereof. and a duct for supplying saidheated and moisturized furnace gases to said kiln chamber forcirculation therethrough,

an adjustable damper coupling said furnace duct to said furnace airintake for internally recirculating through said furnace a selectableportion of said heated furnace gases,

. a first sensor located in said kiln chamber for measuring the dry bulbtemperature of the gases circulating therein,

. a second sensor located in said kiln chamber for measuring the webbulb temperature of said circulating kiln gases,

f. a regulator connected to receive signals from said first and bleportion of said heated furnace gases,

d. temperature sensor means located in said kiln chamber for monitoringthe temperature of said circulating kiln gases, and

e. controller means connected to receive signals from said temperaturesensor means and regulating the operation of said adjustable damper inresponse to heat demand requirements for said furnace.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 614O74 Dated October 19 1971 Inventor) Walker L. Wellford, Jr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Col. 5 line 19, delete "100." (second occurrence);

line 46, "1256 should read l25b;-.

Col. 10, line 21, "an an should read as an;

line 62, "web" should read wet-.

Signed and sealed this Lrth day of April 1 972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSGHALK Attesting Officer Commissionerof Patents QM P0405) (10-591 USCOMM-DC wan-Pea 9 U 54 GOVERNNENYPRINTING OFFICE I," 0-355-33

1. In a direct-fired kiln system, the combination comprising a. a kiln chamber for drying and conditioning material, b. a kiln furnace having an air intake, a burner, and a duct coupling heated furnace gases to the inlet of said kiln chamber, c. spray means in said kiln furnace for injecting water for vaporization into said heated furnace gases, d. intake air control means for adjusting the relative constituents of fresh ambient air and tempering heated air returning from said kiln chamber, the mixture of which constituents is supplied as intake air to said furnace, and e. controller means for regulating the operation of said intake air control means during periods when water is interjected into said heated furnace gases by said spray means, whereby the caloric heat content of said furnace gases is maintained at a level sufficient to effect a rapid and substantially complete vaporization of said water spray therein.
 2. The kiln system of claim 1 further including a passageway provided with an adjustable damper coupling said furnace duct directly to said furnace air intake for internally recirculating within said furnace a selectable portion of said heated furnace gases, the operation of said adjustable damper also being regulated by said controller means.
 3. In a direct-fired kiln system, the combination comprising a. a kiln chamber for drying and conditioning material, b. a kiln furnace having a burner for heating gases and a spray means for injecting water for vaporization into said heated furnace gases for increasing the moisture content thereof, said heated and moisturized furnace gases being thereafter supplied to said kiln chamber for circulation therethrough, c. fan means controlling the rate of flow of said gases through said furnace, and d. controller means for regulating the operation of said fan means during periods when water spray is added to said furnace gases, whereby the caloric heat content of said furnace gases is maintained at a level sufficient to effect a rapid and substantially complete vaporization of said water spray therein.
 4. In a direct-fired kiln system, the combination comprising a. a kiln chamber for drying and conditioning material, b. a kiln furnace having an air intake, a burner, and a duct coupling heated furnace gases to the inlet of said kiln chamber, c. spray means in said kiln furnace for injecting water for vaporization into said heated furnace gases, d. an adjustable damper coupling said furnace duct directly to said furnace air intake for internally recirculating within said furnace a selectable portion of said heated furnace gases, and e. controller means for regulating the operation of said adjustable damper during periods when water is interjected into said heated furnace gases by said spray means, whereby the caloric heat content of said furnace gases is maintained at a level sufficient to effect a rapid and substantially complete vaporization of said water spray therein.
 5. In a direct-fired kiln system, the combination comprising a. a kiln chamber for drying and conditioning material, b. a kiln furnace having an air intake, a burner, a spray means for injecting water for vaporization into heated furnace gases for increasing the moisture content thereof, and a duct for supplying said heated and moisturized furnace gases to said kiln chamber for circulation therethrough, c. fan means controlling the rate of flow of said furnace gases through said furnace, d. intake air control means for adjusting the relative constituents of fresh Ambient air and tempering heated air returning from said kiln chamber, the mixture of which constituents is supplied as intake air to said furnace, e. a passageway provided with an adjustable damper coupling said duct directly to said air intake for bypassing said furnace chamber and internally recirculating within said furnace a selectable portion of said heated furnace gases, f. a pair of sensors positioned in said kiln chamber for monitoring respectively the instantaneous temperature and humidity conditions prevailing therein, and g. controller means connected to receive signals from said sensor pair and acting on said water spray means, said fan means, said intake air control means, and said adjustable damper, whereby the operation of said kiln furnace is regulated so as to restore and maintain desired predetermined temperature and humidity conditions in said kiln chamber.
 6. A kiln system according to claim 5 further including a third sensor positioned in said furnace so as to measure the temperature of said heated furnace gases and connected as a signal input to said controller means, whereby the operation of said fan means, said intake air control means and said damper are regulated so as to reduce the temperature of said heated furnace gases when the signals received from said third sensor indicate that the temperature of said gases is closely approaching a predetermined maximum level.
 7. In a direct-fired kiln system, the combination comprising a. a kiln chamber for drying and conditioning material, b. a kiln furnace having an air intake, a burner, a spray means for injecting water for vaporization into heated furnace gases for increasing the moisture content thereof, and a duct for supplying said heated and moisturized furnace gases to said kiln chamber for circulation therethrough, c. intake air control means for adjusting the relative constituents of fresh ambient air and tempering heated air returning from said kiln chamber, the mixture of which constituents is supplied as intake air to said furnace, d. a first sensor located in said kiln chamber for measuring the dry bulb temperature of the gases circulating therein, e. a second sensor located in said kiln chamber for measuring the wet bulb temperature of said circulating kiln gases, f. a regulator connected to receive signals from said first and second sensors and generating a pair of ''''heat'''' and ''''humidity'''' output signals indicating, respectively, the instantaneous heat and humidity requirements of said circulating kiln gases as determined by comparison of said sensor signals with predetermined settings of said regulator, and g. a controller connected to said regulator containing circuit means which actuate said spray means when increased humidity is needed and which regulate the operation of said intake air control means in response to the greater of the respective magnitudes of said ''''heat'''' and ''''humidity'''' signals received from said regulator.
 8. A kiln system according to claim 7 further including a passageway provided with an adjustable damper coupling said furnace duct to said furnace air intake for internally recirculating through said furnace a selectable portion of said heated furnace gases, the operation of said adjustable damper also being regulated by said controller means in response to the greater of the respective ''''heat'''' and ''''humidity'''' signals received from said regulator.
 9. A kiln system according to claim 7 further including a fan means controlling the rate of flow of said furnace gases through said furnace, the operation of said fan means also being regulated by said controller means in response to the greater of the respective ''''heat'''' and ''''humidity'''' signals received from said regulator.
 10. A kiln system according to claim 7 including a third sensor positioned so as to measure the temperature of said heated furnace gases and connected as a signal input to said circuit Means in said controller whereby, in the event the temperature of said heated furnace gases closely approaches a predetermined safety threshold, said third sensor signal acts an an override on the operation of said controller so as to regulate said furnace air intake means in a manner so as to reduce the temperature of said heated furnace gases.
 11. In a direct-fired kiln system, the combination comprising a. a kiln chamber for drying and conditioning material, b. a kiln furnace having an air intake, a burner, a spray means for injecting water for vaporization into heated furnace gases for increasing the moisture content thereof, and a duct for supplying said heated and moisturized furnace gases to said kiln chamber for circulation therethrough, c. means controlling the rate of flow of said furnace gases through said furnace, d. means for controlling the temperature of the intake air supplied to said furnace, e. a pair of sensors positioned in said kiln chamber for monitoring respectively the instantaneous temperature and humidity conditions prevailing therein, and f. controller means connected to receive signals from said sensor pair and acting on said water spray means, said fan means and said intake air control means, whereby the operation of said kiln furnace is regulated so as to restore and maintain desired predetermined temperature and humidity conditions in said kiln chamber.
 12. In a direct-fired kiln system, the combination comprising a. a kiln chamber for drying and conditioning material, b. a kiln furnace having an air intake, a burner, a spray means for injecting water for vaporization into heated furnace gases for increasing the moisture content thereof, and a duct for supplying said heated and moisturized furnace gases to said kiln chamber for circulation therethrough, c. an adjustable damper coupling said furnace duct to said furnace air intake for internally recirculating through said furnace a selectable portion of said heated furnace gases, d. a first sensor located in said kiln chamber for measuring the dry bulb temperature of the gases circulating therein, e. a second sensor located in said kiln chamber for measuring the web bulb temperature of said circulating kiln gases, f. a regulator connected to receive signals from said first and second sensors and generating a pair of ''''heat'''' and ''''humidity'''' output signals indicating, respectively, the instantaneous heat and humidity requirements of said circulating kiln gases as determined by comparison of said sensor signals with predetermined settings of said regulator, and g. a controller connected to said regulator containing circuit means which actuate said spray means when increased humidity is needed and which regulate the operation of said adjustable damper in response to the greater of the respective magnitudes of said ''''heat'''' and ''''humidity'''' signals received from said regulator.
 13. In a direct-fired kiln system, the combination comprising a. a kiln chamber for drying and conditioning material, b. a kiln furnace having an air intake, a burner, and a duct for supplying heated furnace gases to said kiln chamber for circulation therethrough, c. an adjustable damper coupling said furnace duct directly to said furnace air intake for bypassing said kiln chamber and internally recirculating within said furnace a selectable portion of said heated furnace gases, d. temperature sensor means located in said kiln chamber for monitoring the temperature of said circulating kiln gases, and e. controller means connected to receive signals from said temperature sensor means and regulating the operation of said adjustable damper in response to heat demand requirements for said furnace. 